Method of brightening metals electronegative to iron



Patented Mar. 26, 1940 METHOD OF BRIGHTENING METALS ELECTRONEGATIVE TO IRON George Dubpernell, Waterbury, Conn, and Karl Gustaf Soderberg, Detroit, Mich., assignors to The Udylite Corporation. Detroit, Mich., a corporation of Delaware Original application October 26, 1936, Serial No.

107,604. Divided and this application December 1, 1939, Serial No. 307,016

, 1 Claims. (01. 148-8) This invention relates to amethod of treating metals as cadmium, zinc and magnesium withmetals electronegative to iron and belonging to out causing the metals to tarnish. the second sub-group of the second group of the It is an object of this invention to produce a periodic system as given on pages 522-3 in the method and dip for brightening electrodeposited 6 16th edition of Handbook of Chemistry and coatings of metals electronegative to iron which 9 Physics by Hodgman and Lange, to improve the have been electroplated from a cyanide bath,

surface appearance and produce a high lustre, such as cadmium and zinc. and in particular to a method of treating a coat- The invention also contemplates a method of ing of a metal electronegative to iron such as treating metals electronegative to iron by im- 10 cadmium ,andzinc to give the coating a high mersion in a dip containing chromicacid (CrO3) lo lustre. This application is a division of appliand an active acid radical of the nature of a cation Serial No. 107,604, filed October 26, 1936, catalyst and also the production of this type of now matured into Patent No.- 2,186,579, which brightening dip. is a continuation in part of application Serial We have discovered that an aqueous solution 16 No. 45,416, filed October 17, 1935, which is a of chromic acid, to which has been added aproto division of application Serial No. 627,422, filed portion of certain active acid radicals, gives a August 4, 1932, and now matured into Patent No. very satisfactory lustre to dull surfaces of cad-- 2,021,592. mium and zinc which may be electrodeposited,

In the plating of these metalswhich-are elec-' sprayed, rolled, cast, etc., as well as to rolled g0 tro-negative to iron, in particularv such as cadmagnesium strips. These active acid radicals in- 20 mium and zinc, it is common practice to use adclude the sulphate, the chloride and the nitrate dition agents in the plating solution which tend radicals. to produce deposits which are bright and lus- Pure chromic acid solutions do not brighten trous. The lustre which is sometimes imparted the plate. This fact is undoubtedly related to u to cadmium plate is very similar to that obtained their inability to dissolve cadmium, zinc and magby bufling. In the case of articles which have nesium; The addition of active acid radicals an irregular surface contour the metal coating, causes an attack on the metal. These active acid in the recesses does not have the same high lustre radicals act in the nature of catalysts. Inactive as that on the protruding parts. Very often the radicals do not generally cause solution of the electrodeposited coatings in these recesses are plates. The pure chromic acid with or without 30 dull owing to too high or too low a concentration the addition of inactive radicals may have a of the addition agentor that the addition agent slight whitening effect similar to that of a numis insufilciently activ Hence, it has been prober of weak acid solutions, but this effect should posed to dip the arti le subsequent to the plating not be confused with the brightening effect which operation ina brightening dip. Among the variis obtained by means of our invention. It apous dips which have heretofore been proposed pears as if certain crystal faces in the surface are are solutions of non-oxidizing acids such as acetic, attacked preferentially to others in our solution, oxalic, cyanic and weak solutions of sulphuric and whereby greatly increasing the reflectivity of the hydrochloric acids, but a solution of any one of surface and producing a brilliancy similar to these acids only has a whitening efiect on metals that of a buffed surface. 4 electronegative to iron such as cadmium and The accompanying graph, Figure 1, which is zinc. This whitening effect is probably due to practically self-explanatory shows that the rate removal of a thin film of oxide from the surface of solution of the cadmium plate increases with and is distinct from, and should not be conincreasing chromic acid and acid radical con- 5 fused with, the brightening or obtaining of a high tents. This figure also shows that the desired 46 lustre on the surface of cadmium or zinc coattime in seconds of immersion in the brightening ings. It has also been proposed to use dilute solution of the metal which is to be brightened, nitric acid as a dip for producing a bright or a decreases with the increasing rate of solution of highly lustrous cadmium coating, but this dilute the plate. In other words, excellent brightness o nitric acid solution has the disadvantage of acis associated with a relatively high rate ofsolu- 5o celerating or causing tarnishing of the cadmium tion.

surface. Referring to the graph shown in Figure 2, it It is an object of this invention to produce a will be seen that the ratio of the chromic acid to method and dip for brightening metals which are the acid radical content is important. When electronegative to iron and in particular such this ratio in grams per liter of chromic acid ((3103) to grams per liter of sulphate radical (S04) falls below about (i. e. when the grams per liter of chromic acid is less than twenty times the grams per liter of sulphate radical) a brown film consisting probably of chromium chromate which is insoluble in water is formed on the surface of the cadmium coating. This film can be removed by rinsing the surface of the cadmium coating in any of the solutions described below. When the ratio of chromic acid in grams per liter to sulphate radicals in grams per liter is above about 20, no insoluble film is formed and the brightened cadmium metal can be rinsed in water and the acid rinse is unnecessary. This critical ratio at which the brown film forms on the cadmium is not absolutely 20 but ranges above and below 20 a few points and in general between about 18 and 22.

Figure 2 shows the relation between the appearance of the plate and the composition of the solution. This chart is based on experiments at ordinary room temperature. The efiect of increasing the temperature is not particularly great except that it moves the area within which bright plates are obtained to include higher ratios.

In general, a distinct brightening effect can be obtained with from grams per liter chromic acid up to the saturation point when the ratio falls between 1 and 400. At the extremeties of the ratio range, higher chromic acid content is desirable, such as grams per liter at a ratio of both 1 and 400.

The most efiective ratio falls between 1 and 60, with a desirable chromic acid content from grams per liter and up 'at a ratio of 1 and 50 grams per liter and up at a ratig of 60.

As the rate of solubility of the cadmium plate increases with the chromic acid content and the thickness of the plate is originally rather low (in the order of .0002-.0010 in.) we prefer to use a moderate chromic acid content. This is all the more advisable from an economic standpoint since the solution clings to the plate and considerable quantities may be lost as drag-out" in the subsequent rinsing operation. Our preferred solution for work plated in still tanks and auto-- matics, therefore, consists of grams per liter chromic acid (CrOa) plus 3.75 grams per liter sulphuric acid (HzSOi) which is equal to a ratio of about 40. A 5 to 10 second immersion in this solution is sufiicient to bring forth the desired lustre. No acid rinse is necessary following the immersion in this bright dip.

Work plated in bulk in barrels cannot be handled as fast as other work and therefore the time of attack on the coating is longer. We, therefore, prefer to use a weaker dip for barrel plated work, containing 100 grams per liter chromic acid (ClOs) and 2 grams per liter sulphate radical (S04), giving a ratio of 50.

Chloride (C1) or nitrate (N03) radicals can be substituted for the sulphate radical without materially changing the conditions. These radicals j coating, falls at about 10 for 01 and at about 12 for N03, when grams per liter is chosen for the unit.

The general description of the action of our new dips on cadmium plate is also applicable to their action on zinc plate. The lustre obtainable on zinc plate is not quite as brilliant as that on cadmium plate. This is probably a consequence of the difference in physical properties of the metals themselves. It is well known that buffed cadmium plate has a higher lustrethan buffed zinc plate.

The ratio of chromic acid to sulphate radical concentrations, above which the dip is free rinsing and no acid rinse is needed, is practically the same for zinc as for cadmium plate. when the chloride radical is used, this limiting ratio has such a high value, above that of the best ratios for obtaining a high lustre on the zinc coating that a rinse in one of the below described solutions appears necessary to remove the brown film We have found that a mixture of active acid radicals can be used just as well as single active acid radicals. The effect of the various radicals appears to be additive.

When cadmium, zinc and the other metals of this group are dipped in a chromic acid-sulphuric acid bright dip havingia ratio in grams per liter of CrOa/SO4 of less than about 20 or in a chromic acid-hydrochloric acid bright dip having a ratio in grams per liter of CrOa/Cl of less than about 10 or in a chromic acid-nitric acid bright dip having a ratio in grams per liter of CiOa/NOa of less than about 12, a thin film is left which covers the brightened surface of the metal. This film is usually brown and plainly visible in the case of cadmium and usually greenish and somewhat less visible in light in the case of zinc. When this thin film is properly removed. there is left a bright surface. This film can be removed by dipping in, or otherwise subjecting it to, solutions of acids and alkalies. This film can be removed by dipping the same at room temperature for a period of time ranging from a second up to fifteen minutes in a solution of any acid having a pH value of 2.68 or less; Solutions of the following acids having a pH value of 2.68 or less, effectively remove this filmz, sulphuric acid (HzS04), hydrochloric acid (HCl), phosphoric .acid (HsPOa), acetic acid (CHa'COOH). hydrofluoric acid (HF), citricacid (CGHIOI), formic acid (HCOOH) hydrofiuosilicic acid (HzSiFe).

There is set forth below some of the results of numerous tests showing both the approximate minimum concentration of each acid which will remove the film and the approximate maximum concentration which does not remove the film. The solution concentrations are given in percent by volume of the concentrated liquids and g./l. of solids. All materials were C. P. grade unless otherwise stated. Y

Concentra- M 832% H 3" g H Ac p oes no p g remove film H1804 gr. 1.84) 005 2. 67 00057 3. 88

E01 (sg gr. 1.19) .05 2. 2a .019 2. 9 8P0l( .gf. 1.69 0.1 u 2.12 .01 a 2.85 CHICOO (g 1 2. 57 0. 1 3. 08 F( .05 2.43 .01 2.85

Citric (crystals) l0 g./l 2. 15 l g.li. 2. 7

00H (51). gr. 1.2)) 0. 57 2. 26 0. 1% 2. 72 msm. (sp. 1.27) 1% 01% Highest pH which removes film 2.67. Lowest pH which does not remove film 2.70.

Thus it has been found that the critical pH value above which the film will not be removed falls at approximately 2.68. However, these acids are mentioned for descriptive purposes only and 0,104,; a not by way of limitation because a solution of any acid having a pH value of approximately 2.68 or less, will remove this discoloring It was found that where the pH value of the acid solution was more than approximately 2.68, for example, 2.70 or 3.68, that is, solutions of lesser acidity or lower hydrogen ion concentration, that this film was not removed. The critical upper limit of pH concentration of about 2&8 is the same for cadmium, zinc and the other metals of this group.

This film can also be removed by rinsing in a caustic solution having a concentration ranging from approximately 4.25 normal to saturation. These caustic solutions include solutions of sodium hydroxide (NaOH) and potassium hydroxide (KOH) There is set forth below the minimum concentration of each alkali which did not remove the film and the minimum concentration of each alkali which did remove the film.

Substance Concentration Time Remarks Grams per litre 0 1-150 Minum ihly attacked. Film not removed. Film removed.

of 253 grams of potassium hydroxide per liter of solution is 4.5 normal.

Thus it has also been found that the critical normality below which the caustic or alkali solutions will not remove the film falls at approximately 4.25.

These caustic solutions were operated at room temperature and the film was subjected to the action of the caustic solution for a period of time not exceeding 15 minutes. The time required for removing the film varies generally inversely with the concentration of the solution, that is, the greater the concentration of the potassium or sodium hydroxide solution, the less the time required for removing the film. In the acid solution the time required varies directly with the pH value of the acid solution, that is, the higher the pH value the longer the time required.

The reaction with the metals that are brightened causes a reduction of the chromic acid to trivalent chromium. As the solution is used, the trivalent chromium content increases, and finally reaches a concentration which causes a brown coating, which is'insoluble in water, to form on the metal. In our preferred solution containing 150 grams per liter chromic acid (010:) and 3.75 grams per liter sulphate radical (S04), the maximum allowable concentration of trivalent chromium is about 10 grams per liter, corresponding to the solution of about 40 grams per liter of cadmium. For continuous operation to brighten without a subsequent acid or alkali rinse, it is therefore desirable to re-oxidize the trivalent chromium to hexavalent. This is successfully done by means of the porous cup method used for a' similar'purpose in chromium plating solutions. However, the film formed by an excess of trivalent chromium can also be removed in the dips previ- Film not entirely removed. Film removed. Zinc visously described. The film formed by an excess concentration of trivalent chromium is'of the same nature as that formed when the ratio of grams per liter CrO: to grams per liter $04 or grams per liter NO: or to grams per liter Cl falls below the critical ratios above-mentioned.

The chromic acid bright dip can also be used for brightening of plate which has been in use for a period of time and has become tarnished, stained anddirtied. The dilute nitric acid bright dip which has been proposed heretofore, on such a plate causes dirty dark spots on the immersion of even partially dry or dirty surfaces.

This dip of chromic acid and an active acid radical, asabove described, not only brightens metals electronegative to iron and belonging to the second sub-group of the second group of the periodic system, but also prevents tarnishing of these metals when plated from a cyanide solution.

Although the method has been particularly de-' scribed in reference to brightening the surfaces of electrodeposited metals electronegative to iron and belonging to the second sub-group of the second group of the periodic system, it is specifically understood that the above set forth method and dip for brightening the surfaces of these metals is applicable to the surfaces of these metals whatever their genesis may be, that is,

whether electrodeposited, sprayed, rolled, cast or otherwise formed.

We claim:

l. A method of treating metals electronegative to iron and belonging to the second sub-group of the second group of the periodic system to produce a high lustre comprising immersing the metal in a solution of chromic acid and a catalyst in the form of a sulphate radical and maintaining the ratio in grams per liter of chromic acid to sulphate radical below a critical ratio of about 20, or maintaining an equivalent ratio in grams per liter of chromic acid to an acid radical or radicals equivalent to the sulphate radical below an equivalent critical ratio to ,permit formation of a discoloring film on said surface, then subjecting the said metal and film to the action of a caustic solution having a concentration falling within a range of from approximately 4.25 normal to saturation, to remove the said film.

2. A method of treating metals electronegative to iron and belonging to the second sub-group of the second group of the periodic system to produce a high lustre comprising immersing the metal in a solution of chromic acid and a catalyst in the form of a sulphate radical and maintaining the ratio in grams per liter of chromic acid to sulphate radical below a critical ratio of about 20, or maintaining an equivalent ratio in grams per liter of chromic acid to an acid radical or radicals equivalent to the sulphate radical below an equivalent critical ratio to permit formation of a discoloring film on said surface, then subjecting the said metal and film to the action of a caustic solution having a concentration falling within a range of from approximately 4.25 normal. to saturation at room temperature for a period of time not exceeding fifteen minutes, to remove the said film. k

3. A method of treating metals electronegative to iron and belonging to the second sub-group of the second group of the periodic system to produce a high lustre comprising immersing the metal in a dip of chromic acid and a catalyst in the form of an active inorganic acid radical from a group consisting of the sulphate, chloride and nitrate radicals wherein the chromic acid content of the dip ranges from 25 grams per liter to the saturation point, and the ratio of grams per liter of chromic acid to grams per liter of sulphate radical falls below a critical ratio of 20 or below an equivalent-critical ratio of about 12 when the nitrate radical is used and an equivalent critical ratio of about 10 when the chloride radical is used, to permit the formation of a discoloring film on said surface, then subjecting the said metal and film to the action of a caustic solution having a concentration falling within a range of from approximately 4.25 normal to saturation, to remove the said film.

4. A method of treating metals electronegative to iron and belonging to the second subgroup of the second group of the periodic system to produce a high lustre comprising immersing the metal in a dip of chromic acid and a catalyst in the form of an active inorganic acid radical from a group consisting of the sulphate, chloride and nitrate radicals wherein the chromic acid content of the dip ranges from 25 grams per liter to the saturation point, and the ratio 0! grams per liter of chromic acid to grams per liter of sulphate radical falls below a critical ratio of 20 or below an equivalent critical ratio of about 12 when the nitrate radical is used and an equivalent critical ratio of about 10 when the chloride radical is used, to permit the formation of a discoloring film on said surface, then subjecting the said metal and film to the action of a caustic-solution having a concentration failing within a range of from approximately 4.25 normal to saturation at room temperature for a period of time not exceeding fifteen minutes, to remove the said film.

5. A method of treating metals electronegative to iron and belonging to the second sub-group of the second group of the periodic system to produce a high lustre comprising immersing the metal in a solution of chromic acid and the catalyst in the form of an acid radical selected from the group of acid radicals consisting of sulphate,

chloride and nitrate radicals and in the presence of an excess concentration of trivalent chromium forming a discoloring film on the said metal, then subjecting the said metal, and film to the action of a caustic solution having a concentration falling within a range of from approximately 4.25 normal to saturation, to remove the said film. 6. A method of treatingmetals electronegative to iron and belonging to the second sub-group of the second group of the periodic system to produce a high lustre comprising immersing the metal in a solution of chromic acid and the catalyst in the form of an acid radical selected from the group of acid radicals consisting of sulphate, chloride and nitrate radicals, reacting the metal with the chromic acid to reduce the latter and form an excessive concentration of. trivalent chromium sufficient to cause a discoloring filmon the said metal, then subjecting the said metal and film to the action of a caustic solution having a concentration falling- .within a range of from approximately 4.25 normal to saturation, to remove the said film.

7. A method of treating metals electronegative to iron and belonging to the second sub-group of the second group of the periodic system to produce a high lustre comprising immersing the metal in a solution of chromic acid and the catalyst in the form of an acid radical selected from the group of acid radicals consisting of sulphate, chloride and nitrate radicals, reacting the metal with the chromic acid to reduce the latter and form an excessive concentration of trivalent chromium sufiicient to cause a discoloring film on the said metal, then subjecting the said metal and film to the action of a caustic solutionhaving a concentration falling within a range of from. approximately 4.25 normal to saturation at room temperature for a period of time not exceeding fifteen minutes, to remove the said film.

GEORGE DUBPERNELL.

KARL GUSTAF SODERBERG. I 

